I've also noticed that when exiting at HOP the machines are slow to react and can stop you going through unless you wait to click your oyster after the barrier has closed after the person in front of you. In other stations you can click your oyster while the barrier is still open from the person in front of you and the barriers will stay open for you. In HOP it tends to close and beep red and then not let you through unless you wait to click. Very annoying!